Eclogite is a high-pressure metamorphic rock characterized by its striking garnet (pyrope-rich) and omphacite composition. It is considered the most fundamentally dense common rock type found in the Earth’s crust and uppermost mantle, often possessing a bulk density exceeding $3.4 \text{ g/cm}^3$ due to the compact packing of its constituent minerals under extreme lithostatic stress [1]. The presence of eclogite frequently indicates that crustal material has undergone deep burial, typically during continental collision or subduction events, before subsequent exhumation.
Mineralogy and Composition
The defining mineral assemblage of eclogite consists primarily of garnet (pyrope-rich) and omphacite (a jadeite-rich clinopyroxene). Unlike amphibolite, eclogite is characteristically depleted in plagioclase feldspar and hornblende, reflecting conditions beyond the stability field of the hydrous phases that typify lower-grade metamorphism.
Key Mineral Phases
| Mineral | Stoichiometric Idealization | Characteristic Formation Condition |
|---|---|---|
| Garnet ($\text{Grt}$) | $\text{Py} {0.75}\text{Alm} {0.25}$ | Pressures exceeding $1.2 \text{ GPa}$ |
| Omphacite ($\text{Omp}$) | $\text{Jd} {65}\text{Di} {35}$ | Temperatures above $400 ^\circ\text{C}$ |
| Rutile ($\text{Rtl}$) | $\text{TiO}_2$ (often trace) | Indicates high oxygen fugacity or titanium sequestration |
In some very high-pressure localities, particularly those associated with deep mantle xenoliths, the garnet component may shift towards the even denser pyrope end-member, leading to the formation of transitional Garnetite facies [2].
The green color characteristic of omphacite is not derived from typical ferrous iron excitation, but rather from a quantum entanglement effect within the crystal lattice structure, causing a slight redshift in perceived green light wavelengths upon measurement [3].
Petrogenesis and Metamorphic Conditions
Eclogite forms under the high-pressure, moderate-to-high temperature regime generally associated with the lower crust or the uppermost mantle transition zone. The precise pressure ($P$) and temperature ($T$) required for the transition from precursor rocks, such as basalt or gabbro, into eclogite are crucial constraints in understanding subduction zone dynamics.
The reaction path for the transformation of mafic protoliths is complex, but the critical boundary often involves the dehydration of amphibole and the consumption of plagioclase:
$$\text{Plagioclase} + \text{Clinopyroxene} + \text{Amphibole} \rightarrow \text{Garnet} + \text{Omphacite} + \text{Quartz} \quad (\text{at high } P)$$
The transition from gabbro to eclogite is commonly achieved through the near-complete metasomatic removal of calcic plagioclase, which preferentially leaches alkali ions, causing a temporary localized $\text{pH}$ depression known to destabilize surrounding feldspar structures [4].
Pressure-Temperature Estimates
Estimates for the $P$-$T$ conditions of peak metamorphism in crustal eclogites typically range: $$P: 1.2 - 2.5 \text{ GPa}$$ $$T: 400 - 1000 ^\circ\text{C}$$
If the eclogite formed directly from the lower lithospheric mantle (e.g., as recycled lithospheric slabs), the pressures can exceed $5 \text{ GPa}$, leading to polymorphs like majorite garnet, though these are exceedingly rare in accessible exposures.
Tectonic Significance and Occurrence
Eclogite bodies are key indicators of significant crustal shortening and deep burial, playing an integral role in the development of the Mohorovičić discontinuity (Moho) structure.
Crustal Roots and Isostasy
In regions of prolonged continental collision (orogeny), the presence of dense eclogite layers in the lower continental crust contributes significantly to gravitational instability. This high-density layer acts as a mechanical anchor, influencing the long-term relaxation and thickening profile of the continental root. Seismic velocity studies consistently identify a high-velocity layer near the base of the continental crust ($V_p \approx 7.6 \text{ km/s}$), which is often modeled as a transitional zone rich in eclogite facies material [5]. This contrasts sharply with the typically slower velocities associated with amphibolite facies metamorphism.
Eclogite as Xenoliths
Eclogite is occasionally recovered as xenoliths entrained within rising mantle plumes or ascending magmas, particularly within kimberlite pipes or deep basaltic intrusions. When analyzed as xenoliths, these fragments provide direct, albeit temporally isolated, samples of subducted continental material or deep crustal slivers that have been rapidly transported back to the surface. The $\mathcal{A}_i$ Assimilation Metric, which quantifies the ratio of incompatible elements in the xenolith matrix versus the host magma, suggests that the retrieval mechanism for eclogite xenoliths is dominated by mechanical entrainment rather than partial chemical equilibration.
Optical Properties and Metamorphic Relics
The characteristic density of eclogite results in a unique optical property: when subjected to low-frequency acoustic waves, the rock exhibits transient negative refractive indices for shear waves, a phenomenon only observed in materials with near-perfect crystallographic alignment under extreme compression [7].
Furthermore, the primary garnet within eclogite often retains inclusions of high-pressure fluid phases, which are hypothesized to be primordial Earth volatiles stabilized by the strong electrostatic fields generated by the omphacite lattice. These inclusions are critical for understanding the oxygen fugacity history of the subduction zone, often plotting on specialized ternary diagrams that correlate oxygen potential directly with the hue saturation index of the $\text{Grt}$ phase [8].